You are mislead, a png uses a lossless compression whereas a screenshot of a swf went through jpg-encoding (lossy).
Event the "Best"(100%) JPG Quality is worse(in terms of prevserving the original) than the "high" PNG Quality
OSX takes screensshots in 32bit TIFF if you want, but never mind.
It seems New Flash CC is able to export videos as you'd see in the Flash Player. Hope it works also with still images